Conventionally, a step for manufacturing a sealed battery includes a leak testing step for checking the airtightness of a battery case for the purpose of, for example, prevention of degradation of battery performance caused by ingress of moisture into the battery case (see Patent Literature 1, for example).
Patent Literature 1 discloses a technique as follows:
First, a battery can (i.e., a battery case) is sealed except for a pouring hole, and then, air in the battery can is exhausted through the pouring hole by exhausting means (i.e., the pressure inside the battery can is reduced).
Next, the battery can is connected to an electrolytic solution pot, and then, a difference in pressure between the battery can and the electrolytic solution pot causes an electrolytic solution to be poured into the battery can through the pouring hole. At this time, the inside of the electrolytic solution pot is pressurized with helium gas so that the helium gas is introduced into the battery can through the pouring hole.
Finally, a leak testing step is performed in which the pouring hole is sealed, and then, the quantity of helium gas present in leak gas leaked from the battery can is checked with a helium leakage detector.
If helium gas is introduced when an electrolytic solution is poured similarly to the technique disclosed in Patent Literature 1, the electrolytic solution penetrates an electrode body before a leak testing step is performed, and then, gas present in the electrode body is exhausted to the outside of the electrode body. Accordingly, a density of helium gas inside a battery can is decreased.
The penetrance of the electrolytic solution with respect to the electrode body at this time, namely, a quantity of the exhausted gas varies depending on various periods of time from the introduction of helium gas to the leak testing step.
In other words, in the technique disclosed in Patent Literature 1, the variations in penetrance of the electrolytic solution with respect to the electrode body cause a variation in density of helium gas present in the leak gas (i.e., density of helium gas inside the battery can in the leak testing step).
As shown in FIG. 7, an output value of a helium leakage detector when a predetermined quantity of leak gas leaks from a battery can depends on the density of helium gas present in the leak gas. Specifically, an output value of a helium leakage detector when a predetermined quantity of leak gas leaks from a battery can becomes large in the case where the density of helium gas present in the leak gas is high (see a graph G11 in FIG. 7), whereas an output value of a helium leakage detector when a predetermined quantity of leak gas leaks from a battery can becomes small in the case where the density of helium gas present in the leak gas is low (see a graph G12 in FIG. 7).
In a leak testing step, an inspection threshold T1 needs to be set on the basis of a leakage of the leak gas in the case of the low density of helium gas present in the leak gas.
Consequently, there is a possibility that an output value of a helium leakage detector exceeds the inspection threshold T1 in the case where the density of helium gas present in the leak gas is high, in spite of the leakage of the leak gas that is smaller than a leakage L of the leak gas corresponding to the inspection threshold T1 in the case of the low density of helium gas present in the leak gas (see a range R1 in FIG. 7).
In the case where the density of helium gas present in the leak gas varies similarly to the technique disclosed in Patent Literature 1, the inspection threshold T1 needs to be made small by the variation, and therefore, normal products may be erroneously determined as defective products with a relatively high probability.
As mentioned above, there is a possibility that an erroneous determination rate is increased in the leak testing step in the technique disclosed in Patent Literature 1.